Saturn, Moon, Star to Offer Triple Night Sky Treat

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Look toward the southeast on Thursday night (May 3) about an hour
after sunset, and you will behold a rare and beautiful sight, a
triple conjunction of the moon, the planet Saturn, and the bright
star Spica.

A
conjunction occurs when two or more astronomical objects are
close together in the sky. In reality they are far apart in
space; their closeness is just an effect of perspective. In
astrology such close encounters are supposed to cause serious
effects, but astronomers know that conjunctions are nothing more
than a beautiful sight and a photo opportunity.

Because
Saturn is the farthest planet from the sun visible to the
naked eye, its movements in our sky are very slow. It has been in
the constellation Virgo, close to Spica, for a couple of years
now. The moon, on the other hand, is very close to Earth, so
appears to move relatively quickly across the sky.

Conjunctions like this one provide an excellent opportunity to
see how rapidly the moon moves from night to night. On Thursday
night, the moon will be well to the right of Saturn and Spica.
Twenty-four hours later, on Friday night, it will be well to the
left of Saturn and Spica.

Astronomers measure distances in the sky in degrees, a complete
circle being 360 degrees. The moon takes 27.3 days to complete
its 360 degree trip around the sky, so moves 13.2 degrees in one
night. By comparing the views Thursday and Friday night you can
see this with your own eyes.

The moon will look "full" on both these nights to the naked eye.
A look with a telescope will reveal that it is not truly full,
but has a clear "terminator" or shadow line on its eastern limb.
This shows that it is still in its waxing gibbous phase. True
full moon does not occur until Saturday night at 11:35 p.m. EDT.
This full moon will actually be the largest and brightest of the
year, as it is a " supermoon "
-- a coincidence of full moon falling at the same time as the
moon's perigee, or closest approach to Earth.

Though conjunctions like this have no physical significance, they
do make for excellent photo opportunities.

If you observe an hour after sunset, the moon will appear
impressively large to your naked eye, but will probably
disappoint you in a photograph. This is because the moon's
apparent large size is actually an optical illusion known as "the
moon illusion." To duplicate this illusion in a picture, you need
to resort to a little trick photography. Adjust your zoom lens to
its maximum setting. This will make the moon loom large relative
to foreground objects, and duplicate what the human eye and brain
appear to see.